Recently, development has been made regarding decrease in thickness and weight of an information-processing equipment such as a note board type personal computer. In parallel with this development, a polarizing plate protective film used in a liquid crystal display has been eagerly sought to be increasingly thinner. Cellulose acetate film is generally used in the polarizing plate protective film, however, it has been proved that a simple decrease in thickness of the cellulose acetate film produces various problems.
The cellulose triacetate film used in the polarizing plate protective film contains a plasticizer in order to improve film flexibility or a moisture vapor permeation property. However, a simple decrease in thickness of the cellulose triacetate film increases moisture vapor permeability, minimizes water resistance, and occasionally deteriorates a polarizing film or an adhesive used for preparing a polarizing plate. In order to compensate for the decrease in thickness, it is considered to increase an addition amount of the plasticizer in the film in proportion to the thickness decrease. However, it has been found that a simple increase of the plasticizer addition amount does not sufficiently improve the moisture vapor permeation property causes and produces new problems such as retention property deterioration. The weight of the film decreases at high humidity and high temperature due to volatilization or precipitation of additives such as plasticizers. A property that additives such as plasticizers are retained in the film is referred to as a retention property. Conventional cellulose ester films are low in the retention property, which lowers performance of a liquid crystal display employing the cellulose ester film. There is a technique in Japanese Patent O.P.I. Publication Nos. 11-092574 in which citric ester is employed, a technique in Japanese Patent O.P.I. Publication Nos. 11-114445 in which a pentaerythritol ester or a dipentaerythritol ester is employed, a technique in Japanese Patent O.P.I. Publication Nos. 11-246704 in which glyceride is employed, or a technique in Japanese Patent O.P.I. Publication Nos. 12000-63560 in which diglycerin ester is employed. However, these techniques do not sufficiently improve moisture vapor permeation and deteriorate the retention property.